Description (shell):Shell thick and strong, broadly oval, with prominent umbones. Periostracum typically brittle and peeling readily in largest shells. Anteriorhinge line strongly curved, lunule ill-defined; escutcheon shallow, occupied by a thick, arched, brown or black, ligament. Sculpture of concentric lines and few irregular grooves, growth stages distinct. Right valve with three prominent cardinal teeth and a single posteriorlateral tooth; triangular pit in front of anterior cardinal, surrounded by small knobs and ridges. Left valve with three cardinals and one posterior lateral, anterior cardinal continuous with a series of small ridges and denticulations. Inner surfaces smooth, glossy; adductor scars distinct, posterior one slightly larger than anterior one. Ventral margin crenulate (A. islandica-drawing).Young specimens appear rather different from older ones, the periostracum tending to be smoother and lighter in colour; umbones are relatively less prominent and the shells appear to have a more oval and less tumid shape.

Size:Up to 120 mm long.

Colour:Dull white with a thick periostracum: glossy chestnut-brown in smaller individuals, deep greenish brown to black in large specimens.

Animal:Colour of a creamy tint. The mantle forms two short tubes behind, whose mouths are fringed with delicate tentacles. The long foot is broad at the base, but runs to a sharp point. By the aid of this powerful foot the animal burrows in the muddy sand, but not deeply, as may be gathered from the shortness of the tubes (A. islandica-animal).

Habitat:In sand and muddy sand; offshore, perhaps to the edge of the continental shelf.